Flame-based synthesis is an attractive industrial process for the large
scale generation of nanoparticles. In this aerosol process, a gasifi ed precursor is
injected into a high-temperature turbulent
flame, where oxidation followed by
particle nucleation and other solid phase dynamics create nanoparticles. Precursor oxidation, which ultimately leads to nucleation, is strongly influenced
by the turbulent flame dynamics. Here, direct numerical simulation (DNS) of
a canonical homogeneous flow is used to understand the interaction between
a methane/air flame and titanium tetrachloride oxidation to titania. Detailed
chemical kinetics is used to describe the combustion and precursor oxidation
processes. Results show that the initial precursor decomposition is heavily
influenced by the gas phase temperature field. However, temperature insensitivity of subsequent reactions in the precursor oxidation pathway slow down
conversion to the titania. Consequently, titania formation occurs at much
longer time scales compared to that of hydrocarbon oxidation. Further, only a fraction of the precursor is converted to titania, and a signi cant amount of
partially-oxidized precursor species are formed. Introducing the precursor in
the oxidizer stream as opposed to the fuel stream has only a minimal impact
on the oxidation dynamics. In order to understand modeling issues, the DNS
results are compared with the laminar
flamelet model. It is shown that the
flamelet assumption qualitatively reproduces the oxidation structure. Further,
reduced oxygen concentration in the
near-flame location critically a ffects titania formation. The DNS results also show that titania forms on the lean and rich sides of the
flame. A reaction path analysis (RPA) is conducted.
The results illustrate the di ffering reaction pathways of the detailed chemical
mechanism depending on the composition of the mixture. The RPA results
corroborate with the DNS results that titania formation is maximized at two
mixture fraction values, one on the lean side of the flame, and one on the rich
side. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/23016 |
| Date | 03 February 2014 |
| Creators | Singh, Ravi Ishwar |
| Source Sets | University of Texas |
| Language | en_US |
| Detected Language | English |
| Format | application/pdf |
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